1. Field of the Invention
In the automotive field, throttle valve units are increasingly produced in large batches in the form of plastic injection molded components. For example, such throttle valve units are valves injection molded into the valve housing together with the injection molding process that produces the housing. The throttle valve units that are used in the automotive field are subjected to temperatures between −40° C. and 140° C. so that care must be taken to assure the operational reliability of the formed parts in this temperature range, specifically with regard to gap widths that can be achieved in the injection molding process.
2. Description of the Prior Art
EP 0 482 272 B1 relates to a valve unit, disclosing a valve device and a method for manufacturing a moving valve in a housing that accommodates the moving valve. The valve and the valve housing can be manufactured in one and the same die. The housing is manufactured in a first injection molding step and the disk-shaped valve part is formed into it in a second injection molding step. On the valve part that moves in relation to the housing, sealing sections are provided, which cooperate in a sealing fashion with housing regions of the valve housing. The valve part is preferably of the butterfly type and the valve housing is preferably of the type designed to accommodate a butterfly type valve. The disclosed manufacturing method is capable of significantly reducing the production cost of a valve device for the automotive field. In this embodiment variant, the valve and its housing are positioned transversely in relation to the air flow direction.
U.S. Pat. No. 5,304,336 likewise relates to a method for manufacturing a throttle device. The device contains a moving part and a housing for accommodating the moving part. The moving part and the housing are produced through sequential manufacturing steps of the injection molding process. Preferably, the housing is injection molded in a first process step whereas the part that moves in relation to the housing is produced in another manufacturing step, this moving part being situated in an at least partially closed position. According to the disclosed manufacturing method, a surface of the housing serves as at least a portion of the mold for forming a sealing portion of the movable valve part, thus achieving a very close tolerance between the housing and the valve part that moves in relation to it. According to U.S. Pat. No. 5,304,336 as well, the valve part that moves in relation to the housing is embodied as butterfly-shaped. The housing is preferably of the type that accommodates a butterfly type valve.
The manufacturing methods known from EP 0 482 272 B1 and U.S. Pat. No. 5,304,336 for producing an air-guiding part by means of an injection molding process have the disadvantage that these methods can produce formed parts that may be insufficient in their operational reliability. This is essentially due to an insufficient adjustability and reproduction precision of required gap widths in the shaft bearings and in the gas passage bore in devices manufactured in this way. The methods described above do not offer the necessary capacity for deliberately influencing the gap width by means of machine setting parameters in the forming, i.e. during the injection molding process, in order to achieve a definite air leakage quantity in the closed position of the valve. From one production cycle to the next, the required gap widths cannot be achieved with a sufficient degree of reproducibility to attain a definite leakage air quantity in the closed position of the valve. It is only permissible for the precision or uniformity of such gaps in valves to vary within a range of a few μm. This is of considerable importance in the automotive field in which such air-guiding parts are subjected to a larger temperature range within temperatures of between −40° C. and 140° C. (engine operation temperature in the cylinder head region). Due to a close interconnection between the temperature of the forming die and the cycle time of the injection molding process according to the above-cited manufacturing methods, the required degree of precision cannot be achieved by means of the cavity provided in the forming die. This is particularly true when, according to the methods in the embodiments described above, partially crystalline or amorphous thermoplastic high temperature plastics are used for the above-indicated temperature range for engine compartment applications. According to the manufacturing methods known from EP 0 482 272 B1 and U.S. Pat. No. 5,304,336, it is not possible to react flexibly enough to process fluctuations, e.g. property fluctuations in the molding compounds during forming, i.e. during the production process that includes the injection molding process. The fluctuations described have an impermissibly powerful influence on the quality of the throttle devices produced.